For example if the person was not otherwise posting all the time or being annoying/bothering? Would too little updates be the answer, or not being close friends, or something else?
Have you been unfriended?
I have been unfriended some time ago by several people and was surprised, I didn't know the reason. We did not see each other in a while but we shared a good memory together. Anyway, not sure why or why they even accepted me in the first place?